Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as user equipments, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node. A cell is the geographical area where radio coverage is provided by the access node.
The access node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more access nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. An access node is an entity that controls one or more transmission points. The access node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each access node may support one or several communication technologies. The access nodes communicate over the air interface operating on radio frequencies with the terminals within range of the access node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Every Transmission Time Interval (TTI), every ms in LTE, a transport format is selected when transmitting. The transport format defines a Modulation and Coding Scheme (MCS) and also transmission rank when transmitting single user Multiple Input Multiple Output (MIMO). The transmission rank defines the number of layers transmitted with parallel data streams. Link adaptation selects the transport format as a trade-off between that the transmission can be received and decoded by the terminal with a high probability given the radio conditions at the same time containing as many user data bits as possible. Downlink link adaptation is based on Channel State Information (CSI) reporting. CSI refers to known channel properties of a communication link.
This information describes how a signal propagates from a transmitter to a receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI makes it possible to adapt transmissions to current channel conditions, which is crucial for achieving reliable communication with high data rates in multi antenna systems.
A user equipment measures on downlink reference symbols and predict best rank, Modulation and Coding Scheme (MCS). The user equipment may further select a precoder. Proposed rank is reported as Rank Indicator (RI) and proposed MCS as Channel Quality Indicator (CQI). CQI is reported as quantized efficiency. E.g. for transmission mode 3, rank and CQI is reported, for transmission modes 4, 9, 10: rank, CSI and PMI is reported. CSI reporting is typically configured to be periodic. The choice of reporting period is a trade-off between outdated CSI information and uplink radio resources used for signaling.
The CQI may be noisy and is outdated when used in a base station such as an eNB for MCS selection. An outer-loop is used to adapt to the impact of this and compensate for channel changes and user equipment speed. The outer-loop is typically a Block Error Rate (BLER)-based jump algorithm that targets a certain Hybrid Automatic Repeat Request (HARQ) BLER, e.g. 10%, adjusting with a margin in dB for the CQI inaccuracy. The outer-loop also adapts to other measurement noise such as user equipment vendor implementation differences and measurement errors.
Co-ordinated link adaptation is a promising feature. With good backhaul the MCS selection may be improved with very small or no radio resource cost resulting in improved spectrum efficiency. Within the same base station the interference changes may be predicted every TTI. In a deployment with micro RRUs on the same base station as an overlaying macro RRU, fast and rather accurate MCS changes may be foreseen before measured and reported by the user equipment.
A RRU is a remote radio unit, sometimes also called a radio head, controlled by a base station over an electrical and/or optical interface. One interface that may be used between controlling base station and RRU is Common Public Radio Interface (CPRI). The RRUs may be with different power capability similar to base stations, micro RRU similar as a micro eNB around 2×5 W and macro equal similar as a marco eNB 2×20 W or more.
CSI Reference Signals (CSI-RS) are introduced in LTE release 10. CSI-RS may be used in shared cell ID deployment for CSI reporting.
Shared cell ID also referred to as combined cell is when two or more cells, which may be the same or different access nodes, are combined into one cell given the same cell ID and avoids performing handover between the access nodes. The separate access nodes, previous cells, are still scheduled as separate cells. To enable good link adaptation taking the radio condition and interference within the combined cell into account orthogonal CSI-RS are configured and transmitted from each access node.
CSI-RS improves the measurement accuracy and thereby also the link adaptation within the combined cell. CSI-Interference Measurement (IM) is introduced in LTE release 11 which improves the interference prediction by allocating empty symbols in the LTE time-frequency resource grid in a cell which improves interference measurement by the user equipment since there is then only interference energy present on these symbols.
The user equipment may be configured to send Sounding Reference Symbols (SRS). SRS may be used for uplink Radio Resource Management (RRM) features and uplink link adaptation.
In 3GPP Draft R1-094553, —20091109—3rd Generation Partnership Project (3GPP), Mobile Competence Centre; 650, route des Lucioles; F-06921 Sophia-Antipolis Cedex; France, Jeju; 20091109, a method relating to applicability of channel reciprocity in LTE-A downlink transmission” is shown.
R1-094553 discloses the support for single cell transmission in LTE-Advanced. Signal to Interference-plus-Noise Ratio (SINR) is calculated and used for determining a MCS based on uplink SRS for determining downlink channel quality.
This document relates to a single cell and a single transmission point, and focus on Multi User-Multiple input Multiple Output (MU-MIMO) and multipath channel estimation. In R1-094553, MCS may be selected based on Downlink channel correlation matrix R1 and R2, precoding matrix W1 and W2.
For Link Adaptation:
Outer-loop is a slow process which cannot capture fast interference variations.
CQI is delayed and outdated when used. It can only follow slowly varying interference variation and not bursty interference caused by short packet data bursts.
CQI is not very reliable. The CQI reporting is dependent on UE implementation. There can be large differences how an interference step impact on CQI and is related to true receiver performance.
Coordinated link adaptation although promising, is very challenging based on the available delayed and noisy UE measurement and reporting.